1. Field of the Invention
The present invention relates to technology of, upon the occurrence of an error in a transmission code, presuming the error pattern and correcting such error in an Adaptive Differential Pulse Code Modulation (ADPCM) coding system utilized for voice communication such as digital mobile communication.
2. Description of the Related Art
In a voice signal coding processing by the ADPCM system provided for in KDD Advisory Committee s Recommendation G. 726, a current input signal is predicted from a past input signal, and the difference between the predicted value and an actual value is quantized and transmitted as an ADPCM code. When a radio line is used in voice communications using an ADPCM coding system, an error transmission of a code may occur due to the influence of a channel interference or fading, etc. Upon the occurrence of an error transmission of an ADPCM signal, a click noise (allophone) occures when an ADPCM code is converted by an ADPCM decoder into a PCM code, therefore, in such case of this voice signal coding processing being utilized for digital mobile communications, etc., a device for compensating on a receiving side such error in a transmission code will be required.
Japanese Patent Laid-Open Hei 8-223126, for example, describes a device for improving voice signal in quality as shown in FIG. 2. In FIG. 2, a device for improving voice signal in quality 21 is constructed comprising a sample presumption replacing circuit. The sample presumption replacing circuit 22 receives from a sending set an ADPCM signal via a radio line as well as receives a frame error detecting signal. The frame error detecting signal, when an error occurs in the corresponding frame, outputs “1,” and when an error does not occur, “0.” The sample presumption replacing circuit 22, when the frame error detecting signal is “1,” presumes the status before the occurrence of the error in such code and produces a code, and then replaces such code with the received code. The replacement of code is performed based on the probability distribution of the error pattern of an ADPCM code.
However, as the presumption of an error in a received ADPCM code in the above prior art is performed based on the probability distribution of an error pattern previously calculated, the replacement of a received ADPCM code becomes fixed and an appropriate error correction is not always performed.
Considering that, upon the occurrence of a click noise, the average value for a short interval of an absolute value of the received ADPCM code substantially varies, it would be possible that, when a click noise is detected in such received ADPCM code, a voice signal is improved in quality by presuming the error pattern of the ADPCM code and correcting such error so as to lessen the variation in the average value for a short interval of the absolute value of the received code.